1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new cathode electrode design for a secondary electrochemical cell. The cathode electrode is comprised of a sacrificial piece of alkali metal sandwiched between two current collectors in turn supporting two structures of cathode active material opposite the alkali metal. Upon cell activation, the alkali metal is completely consumed in the cathode active structures. This enables the cathode active material to be those that are not typically used in rechargeable cells. A preferred alkali metal is lithium.
2. Prior Art
Rechargeable or secondary electrochemical cells shuttle alkali metal ions, such as lithium ions, between the anode electrode and the cathode electrode. There are three possible sources of alkali metal ions in a secondary cell. The least utilized is the electrolyte because removal of alkali metal ions from the electrolyte results in loss of its critical ion-conductive properties. The negative or anode electrode is another possible source of alkali metal ions. This requires use of an alkali metal or alloy thereof as the negative electrode. The problem is that poor cycling and safety characteristics limit the use of alkali metals as the negative electrode active material in rechargeable cells.
Consequently, the positive electrode is the source of alkali metal ions in almost all commercial non-aqueous rechargeable cells. This limits the choice of cathode active materials to those that contain alkali metal ions, such as LiCoO2, LiNiO2 and LiMn2O4. However, these materials have specific drawbacks such as low capacity, relatively high cost, insufficient rate capability, poor safety and poor cycleability. Alternate cathode active materials that are not typically used in rechargeable cells because they cannot be acceptably prepared to contain alkali metal ions are better suited to many applications. For example, some alternate cathode active materials have greater pulse capability than the typically used lithiated active materials. Pulse or rate capability is important when the cell is intended to power an implantable medical device such as a cardiac defibrillator. The challenge is, therefore, to incorporate alternate cathode active materials into rechargeable cells without sacrificing electrolyte conductivity, cycling efficiency and safety.
In that light, the present invention relates to rechargeable cells including a positive electrode comprising a “sacrificial” piece of alkali metal short-circuited to an ion-deficient cathode active material. The alkali metal provides alkali metal ions for the cathode electrode of the rechargeable cell. This allows for the effective utilization of cathode active materials that previously could not be incorporated into rechargeable electrochemical cells.